This invention relates in general to control devices for electromagnetic clutches and in particular to a microprocessor based control system for regulating the engagement and disengagement of an electromagnetic clutch.
Clutches are well known devices which selectively connect an input shaft to an output shaft for rotation together. In some clutches, an electromagnet is used to effect this engagement and disengagement. Electromagnetic clutches typically include a rotor and an armature, both of which are generally annular in shape and are supported for rotation about a common axis. The armature is usually connected to the input shaft of the clutch, while the rotor is usually connected to the output shaft. Normally, the armature is spaced apart from the rotor by a relatively small air gap. In this position, the clutch is disengaged, and there is no driving connection between the armature and the rotor. However, the armature can be moved axially into frictional engagement with the rotor. In this position, the clutch is engaged such that the armature and the rotor (and, therefore, the input and output shafts) rotate together.
To effect movement of the armature, and thereby cause the clutch to be engaged, a coil of wire is provided. The coil is generally carried on or supported by a stationary core located adjacent to the rotor. To engage the clutch, an electric current is passed through the coil. The flow of current causes the coil to generate a magnetic field which attracts the axially movable armature toward the axially stationary rotor. When the armature has travelled completely through the air gap, it frictionally engages the rotor as described above, thus connecting the input and output shafts for rotation together. To disengage the clutch, the flow of electric current through the coil is discontinued. In the absence of the magnetic field, a return spring moves the armature out of frictional engagement with the rotor.
As mentioned above, a magnetic field is generated by the coil when the electric current begins to flow therethrough. As the magnitude of the current increases, the intensity of the magnetic field also increases. When the magnetic field intensity is sufficiently large, the armature will begin moving axially toward the rotor. If the magnitude of the electric current is not closely controlled thereafter, the velocity of the armature will increase as it travels toward the rotor, resulting in almost instantaneous engagement of the clutch. Such abrupt clutch engagement has several undesirable side effects, such as the rapid loss of speed of the input shaft, the creation of undue shocks and strains, and the generation of unpleasant noise.
Many control devices are known for avoiding these abrupt engagement problems by regulating the electric current applied to the coil such that armature engages the rotor in a relatively slow and controlled manner. These devices are often referred to as soft start clutch controllers. While known controllers are generally effective for causing relatively soft engagement of the armature with the rotor, they have been found to suffer from several drawbacks. First, they are usually not responsive to external conditions for automatically controlling the operating condition of the clutch (i.e., automatically disengaging the clutch under certain conditions). Second, they do not adjust the magnitude of electric current supplied to the coil in response to changing operating conditions, such as speed, ambient temperature, and the like. Last, they are not readily manually adjustable to compensate for such changing operating conditions. Thus, it would be desirable to provide an improved soft start controller for an electromagnetic clutch which is capable of performing these tasks.